1. Field of the Invention
The present invention relates to a communication apparatus for code-division multiplexing a transmission path by using a spread spectrum communication system in order to increase the transmission capacity.
2. Related Background Art
Hitherto, in a spread spectrum (SS) communication system, a code-division multiple communication system which increases the information transmission speed by multiplexing codes utilizing plural spread codes having low mutual correlative characteristics has been considered. This system has been remarked as one method for conducting a high-speed information transmission within a limited band.
FIGS. 3A and 3B are block diagrams showing a structural example of a code-division multiple communication apparatus in such a conventional spread spectrum communication system. FIG. 3A indicates a transmission unit and FIG. 3B indicates a reception unit.
First, transmission information 31 is converted into n parallel data 33 (1 to n) by an S/P (serial-parallel) converter 32 in the transmission unit shown in FIG. 3A. Each of the parallel data 33 (1 to n) is modulated by each of different spread codes of an SS modulators 34 (1 to n). N spectrum spread SS signals 35 (1 to n) are added by an adder 36 to become a code-division multiple signal 37.
In the reception unit shown in FIG. 3B, a code-division multiple signal 38 is distributed into n code-division multiple signals 40 (1 to n) by a power distributor 39. The distributed multiple signals 40 (1 to n) are inversely spread by SS demodulators 41 (1 to n) to be demodulated to n data 42 (1 to n). The demodulated data 42 (1 to n) are converted into serial data by a P/S (parallel-serial) converter 43 to obtain a demodulation signal 44.
In this way, a high-speed transmission can be achieved while an occupied bandwidth is fixed by converting the transmission information into the parallel data 33 (1 to n) and utilizing a code-division multiple system.
In the transmission unit, the spectrum spread signals 35 (1 to n) are synthesized by utilizing a linear adder 36 for code-division multiplexing. In case where the transmission information 31 is converted into the n parallel data 33 (1 to n), the value of the code-division multiple signal 37 becomes available within the range of 0 to n. In a reception unit, the SS demodulators 41 (1 to n) which enable to execute an inverse spread and treat the value of 0 to n as inputs are required.
However, the value of 0 to n which appear in a code-division multiple signal are not uniformly distributed and it is not efficient to maintain a dynamic-range for an entire area of 0 to n. Also, since useless portions are required on circuit arrangement, simplification or mintaturization of a communication apparatus is prevented.